Parallelogram cell design for high speed vertical channel 3D NAND memory

ABSTRACT

Roughly described, a memory device has a multilevel stack of conductive layers. Pillars oriented orthogonally to the substrate each include series-connected memory cells at cross-points between the pillars and the conductive layers. String select lines (SSLs) are disposed above the conductive layers, and bit lines are disposed above the SSLs. The pillars are arranged on a regular grid having a unit cell which is a non-rectangular parallelogram. The pillars may be arranged so as to define a number of parallel pillar lines, each having an acute angle θ&gt;0° relative to the bit line conductors, each line of pillars having n&gt;1 pillars intersecting a common one of the SSL. The arrangement permits higher bit line density, a higher data rate due to increased parallelism, and a smaller number of SSLs, thereby reducing disturbance, reducing power consumption and reducing unit cell capacitance.

CROSS-REFERENCE TO OTHER APPLICATIONS

This application is a Continuation-In-Part of U.S. application Ser. No. 14/157,550, filed 17 Jan. 2014, entitled THREE-DIMENSIONAL SEMICONDUCTOR DEVICE, incorporated by reference herein.

This application also makes reference to U.S. application Ser. No. 14/582,963, filed 24 Dec. 2014, entitled TWISTED ARRAY DESIGN FOR HIGH SPEED VERTICAL CHANNEL 3D NAND MEMORY, by inventor Shih-Hung Chen, incorporated by reference herein.

BACKGROUND

1. Field of the Invention

The present invention relates to high density memory devices, and particularly to memory devices in which multiple planes of memory cells are arranged to provide a three-dimensional 3D array.

2. Description of Related Art

As critical dimensions of devices in integrated circuits shrink to the limits of common memory cell technologies, designers have been looking to techniques for stacking multiple planes of memory cells to achieve greater storage capacity, and to achieve lower costs per bit. For example, thin film transistor techniques are applied to charge trapping memory technologies in Lai, et al., “A Multi-Layer Stackable Thin-Film Transistor (TFT) NAND-Type Flash Memory,” IEEE Int'l Electron Devices Meeting, 11-13 Dec. 2006; and in Jung et al., “Three Dimensionally Stacked NAND Flash Memory Technology Using Stacking Single Crystal Si Layers on ILD and TANOS Structure for Beyond 30 nm Node,” IEEE Int'l Electron Devices Meeting, 11-13 Dec. 2006, incorporated by reference herein.

Another structure that provides vertical NAND cells in a charge trapping memory technology is described in Katsumata, et al., “Pipe-shaped BiCS Flash Memory with 16 Stacked Layers and Multi-Level-Cell Operation for Ultra High Density Storage Devices,” 2009 Symposium on VLSI Technology Digest of Technical Papers, 2009, incorporated by reference herein. The structure described in Katsumata et al. includes a vertical NAND gate, using silicon-oxide-nitride-oxide-silicon SONOS charge trapping technology to create a storage site at each gate/vertical channel interface. The memory structure is based on a column of semiconductor material arranged as the vertical channel for the NAND gate, with a lower select gate adjacent the substrate, and an upper select gate on top. A plurality of horizontal word lines is formed using planar word line layers that intersect with the columns, forming a so-called gate all around cell at each layer.

FIG. 1 is a horizontal cross-section of a column of a pipe-shaped BiCS flash cell, such as described in the Katsumata et al. publication, at the level of a word line. The structure includes a pillar 15 having a center core 110 of semiconductor material which extends vertically through a stack of word line layers. The core 110 may have a seam 111 through the middle that arises from the deposition technique. A dielectric charge trapping structure comprising for example a first layer 112 of silicon oxide, a layer 113 of silicon nitride and a second layer 114 of silicon oxide (referred to as ONO), or another multilayer dielectric charge trapping structure surrounds the core 110. A gate all-around word line is intersected by the pillar. A frustum of the pillar at each layer combines with the gate all-around word line structure at that layer, to form a memory cell.

FIG. 2 is a perspective view of a 3D semiconductor device. It comprises a multilevel stack of word line conductive layers 11, each parallel to the substrate 10; a plurality of pillars 15 oriented orthogonally to the substrate, each of the pillars comprising a plurality of series-connected memory cells located at cross-points between the pillars and the conductive layers; and a plurality of string select lines (SSLs) 12 oriented parallel to the substrate and above the conductive layers 11, each of the string select lines intersecting a respective row of the pillars. Each intersection of a pillar and a string select line defines a select gate of the pillar. The structure also includes a plurality of parallel bit line conductors 20 in a layer parallel to the substrate and above the string select lines. Each of the bit line conductors superposes a respective column of the pillars, and each of the pillars underlies one of the bit line conductors. The pillars may be constructed as described above with respect to FIG. 1.

FIG. 3 is a top view of a portion of the structure of FIG. 2. It can be seen in both figures that the word line conductive layers 11 intersect only some of the pillars in the overall structure; the word line conductive layer 11 defines a block of memory cells. Thus to read data from a particular block of the memory, control circuitry activates a word line 11 to select a block of cells and a particular layer of the stack, and further activates a string select line 12 to select a particular row. A lower select gate (not shown) is activated as well. A row of cells is then read out in parallel via the bit line conductors 20 into a page buffer (not shown). (“Activate”, as used herein, means to apply a particular bias so as to give effect to the connected cells or switches. The bias may be high or low, depending on the memory design.) Depending on the product specification and design, the page buffer may hold two or more rows of data, in which case a full page read operation would involve successive activation of two or more SSLs 12.

While 3D stacking memory structures hold the promise of greatly increased memory density, they also introduce significant process challenges because, among other things, of the need to etch very deep holes through many layers. Such deep holes have to be made wider, and have to be placed at greater center-to-center distance from each other laterally, in order to meet process windows. As fabrication processes improve, capacity may be increased not only by increasing the number of word line planes in the stack, but also by reducing the spacing between the pillars. FIG. 4 is a top view of a scaled down structure in which both the number of bit lines 20 in a block and the number of SSLs 12 in the block have been increased. Not only does the cost decrease, but increased read/write data rate can be achieved as well because the larger number of bit lines 20 means increased parallel operation. On the other hand, the increased number of SSLs 12 means more cells will suffer Vpass disturb due to word line select. Unit cell capacitance also increases with the number of SSLs 12, thereby increasing power consumption and slowing device operation.

Increasing bit density by increasing the number of word line conductive layers 11 in the stack also has downsides, even aside from the expected process challenges of increased numbers of layers. In FIG. 2 it can be seen that a typical arrangement has a stepped contact structure to the word line conductive layers 11. Deep etches are made through the structure in order to form contacts 22 to connect the conductive layers 11 to metal interconnects 24 above. These contacts 22 are also shown symbolically in the top view of FIG. 4. In a typical design, the number of rows of pillars 15 in a block is at least as great as the number of contacts 22, and hence memory layers. See, for example, Komori, Y., et. al., “Disturbless flash memory due to high boost efficiency on BiCS structure and optimal memory film stack for ultra high density storage device,” Electron Devices Meeting, 2008, IEDM 2008, IEEE International, vol., no., pp. 1-4, 15-17 (December 2008) at 2, incorporated herein by reference. Thus increasing the number of memory layers also increases the number of SSLs 12, and again increases power consumption and slows device operation.

An opportunity therefore arises to create robust solutions to the problem of increasing bit density of 3D memory structures while reducing the negative impacts that such increases tend to cause. Better chip yields, and denser, and more powerful circuits, components and systems may result.

SUMMARY

Roughly described, the present technology provides a memory device having a multilevel stack of conductive layers oriented parallel to a substrate. Pillars oriented orthogonally to the substrate each include series-connected memory cells at cross-points between the pillars and the conductive layers. String select lines (SSLs) are disposed above the conductive layers, each intersection of a pillar and an SSL defining a respective select gate of the pillar. Bit lines are disposed above the string select lines. The pillars in the plurality of pillars are arranged on a regular grid having a unit cell which is a non-rectangular parallelogram. The pillars may be arranged so as to define a number of parallel pillar lines, the pillar lines having an acute angle θ>0° of intersection with the bit line conductors, each line of pillars having exactly n>1 pillars all intersecting a particular common one of the string select lines. The arrangement permits a higher density of bit lines, thereby enabling a higher data rate due to increased parallel operation. It also enables a smaller number of SSLs, thereby reducing disturbance, reducing power consumption and further improving data rate by reducing unit cell capacitance.

The above summary of the invention is provided in order to provide a basic understanding of some aspects of the invention. This summary is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later. Particular aspects of the invention are described in the claims, specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with respect to specific embodiments thereof, and reference will be made to the drawings, in which:

FIG. 1 illustrates a horizontal cross-section of a column of a pipe-shaped BiCS flash cell.

FIG. 2 is a perspective view of a 3D semiconductor device.

FIG. 3 is a top view of a portion of the structure of FIG. 2.

FIG. 4 is a top view of a portion of the structure of FIG. 2, scaled down to permit more bit lines and SSLs.

FIG. 5 is a top view of an example array of pillars in a conventional 3D structure such as that shown in FIGS. 2 and 4.

FIGS. 6, 7, 9 and 10 are top views of arrays of pillars in a 3D structure according to an aspects of the invention.

FIGS. 8A-8D (collectively FIG. 8) illustrate a unit parallelogram cell in several variations.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.

FIG. 5 is a top view of an example array of pillars in a conventional 3D structure such as that shown in FIGS. 2 and 4. Each dot in FIG. 5 represents the lateral position of a corresponding pillar 515. As used herein, “lateral” dimensions refer to dimensions of the structure which are parallel to the substrate (e.g. dimensions labeled X and Y in FIGS. 1, 2, 3 and 4). The structure includes all the other elements shown in FIG. 2, but most of those elements have been omitted from FIG. 5 for clarity of illustration. In particular, the structure represented by FIG. 5 includes the multilevel stack of conductive layers 11, each of the layers being oriented parallel to the substrate. The plurality of string select lines 512 (one of which is shown in FIG. 5) is oriented parallel to the substrate and above the conductive layers, the string select lines being rectangular and having a side oriented in the Y dimension as shown in FIG. 5. As used herein, layers which are termed “above” or “below” other layers, can in various embodiments be separated from such other layers by one or more intervening layers. The same interpretation is intended for layers being described as “superposing”, “underlying” or “over” another layer.

Each of the string select lines intersects a respective distinct subset of the pillars 515, and each of the intersections of a pillar and a string select line defines a respective select gate of the pillar 515. A plurality of parallel bit line conductors 520 (one of which is shown in FIG. 5) extending in the X dimension in FIG. 5 are disposed in a layer parallel to the substrate and above the string select lines, each of the bit line conductors superposing a respective column of the pillars 515. In addition, each of the pillars 515 underlies one of the bit line conductors. Each of the pillars 515 is oriented orthogonally to the substrate (vertically, in the Z dimension as shown in FIG. 2), and includes a plurality of series-connected memory cells located at cross-points between the pillars and the conductive layers. In one embodiment, the lateral cross-section of a column is as illustrated in FIG. 1.

In the arrangement of FIG. 5, it can be seen that the pillars 515 in the array pillars are arranged on a regular grid having two lateral dimensions X and Y. The X dimension is parallel to the bit line conductors, and the Y dimension is orthogonal to the bit line conductors. As used herein, a “regular grid” or “regular array” is a grid (array) which can be divided into adjacent unit cells which collectively fill the array, and all of which have the same shape and size. In FIG. 5 the unit cell is a square and an example is shown as square ABDC. While the grid itself has numerous pillars, the boundaries of the grid, as the term “grid” is used herein, need not have any regularity in a particular embodiment.

As used herein, a “unit cell” in a regular grid is defined as a parallelogram whose four vertices are located at four pillars of the grid. In FIG. 5, for example, the unit cell parallelogram is defined by pillars A, B, C and D. As used herein, the unit cell is defined by starting with a pillar A, then choosing as pillar B a pillar in the grid which is “nearest” to pillar A in a dimension perpendicular to the bit lines. Pillar C is then chosen as a pillar in the grid which is non-collinear with pillars A and B but which is otherwise “nearest” pillar A in the grid, and pillar D is chosen as the pillar located at the fourth vertex of the parallelogram. As used herein, unless otherwise stated, the “distance” between pillars refers to the Euclidean center-to-center distance between the pillars. In addition, the “distance in a particular dimension” between two pillars, as used herein, is the difference between the coordinates of the two pillars in that dimension; ignoring the coordinates in the other dimensions. For example, in FIG. 5, the distance between pillars A and B is d, as is the distance between pillars A and C. The “distance” (i.e. Euclidean distance) between pillars B and C is d√2, but the “distance in the Y dimension” between pillars B and C is d. Also as used herein, the “nearest” pillar to a given pillar is a pillar having the shortest distance to the given pillar. If more than one pillar has the same shortest distance to the given pillar, then any one of them qualifies as being the pillar “nearest” to the given pillar.

In the grid of FIG. 5, the unit cell is a square. As used herein, the term “square” is a special case of the term “rectangle”, in that a square is a rectangle whose four sides all have equal length. Similarly, a “square” is also a special case of a “rhombus”, in that a square is also a rhombus whose four angles are right angles. Furthermore, squares, rectangles and rhombuses are all special cases of the term “parallelogram”. A rectangle is a parallelogram whose four angles are right angles; a rhombus is a parallelogram whose four sides have equal length, and a square is a parallelogram whose four angles are right angles and whose four sides have equal length. Thus the square ABDC in FIG. 5 can equally be called a rhombus, a rectangle, and a parallelogram.

FIG. 6 is a top view of an array of pillars in a 3D structure according to an aspect of the invention. As with FIG. 5, each dot in FIG. 6 represents the lateral position of a corresponding pillar 615. The structure includes all the other elements seen in FIG. 2, though most have been omitted from FIG. 6 for clarity of illustration. Five string select lines 612-1 through 612-5 (collectively 612) are shown, as are eight bit line conductors 620.

As with the structure of FIG. 5, each of the string select lines 612 in FIG. 6 intersects a respective distinct subset of the pillars 615, the intersections defining select gates. Similarly, each of the bit line conductors 620 superposes a respective column of the pillars 615, and each of the pillars 615 underlies one of the bit line conductors 620. In the structure of FIG. 6, however, while the pillars still form rows oriented orthogonally to the bit lines, the pillars in each such row intersects only every other bit line, and alternating rows are shifted so as to intersect alternating sets of the bit lines. FIG. 7 is another illustration of the arrangement of FIG. 6, except that the pillars 615 are shown more in accordance with FIG. 1. Also fewer pillars 615, bit lines 620, and SSLs 612 are shown. In both figures a parallelogram unit cell ABDC is shown.

The shifting of alternating rows of pillars provides two advantages. First, if d is the distance between the pillars in the direction perpendicular to the bit lines, then the bit lines 620 can be formed on a narrower pitch p=d/2. This permits a higher density of bit lines without any reduction needed in the spacing d between adjacent pillars in the grid. Second, the number of SSLs 612 is reduced because each SSL 620 has been made wide enough (in the dimension parallel to the bit lines 620) to intersect two rows of pillars. Stated differently, each SSL 620 is wide enough to intersect all four pillars of a unit cell. For example, in each of FIGS. 6 and 7, one SSL 612 intersects all four pillars of the unit cell ABDC. And despite intersecting two rows of pillars, each intersection of one of the string select lines 612 and one of the bit line conductors 620 still uniquely enables a single one of the pillars in the grid. That is, activation of one word line conductor 11 and one SSL 612 still selects only a single memory cell to each bit line conductor 620. Thus the parallelogram grid of FIGS. 6 and 7 achieves both a higher density of bit lines 620, thereby enabling a higher data rate due to increased parallel operation, and also a lower number of SSLs, thereby reducing disturbance, reducing power consumption and further improving data rate by reducing unit cell capacitance.

FIGS. 8A-8D (collectively FIG. 8) illustrate some of the benefits of the parallelogram cell arrangement. FIG. 8A illustrates a unit cell in a conventional grid such as that of FIG. 3. In this example the center-to-center distance between pillars A and B is the same as the center-to-center distance between pillars C and D, and is given roughly by the diameter of the pillar plus the minimum gate thickness G. But because of the extra spacing S between the two SSLs 812-1 and 812-2 (collectively 812) as required by design rules, and because of a more severe edge gate rule G, the distance in the bit line direction, between pillars A and C and between pillars B and D, must be larger than the distance between pillars A and B and between pillars C and D.

In FIG. 8B the upper row of pillars has been shifted to the right by one-half the distance between them. The cell is now a non-rectangular parallelogram. Since the bit lines 820 do not have to be as wide as SSLs or even as wide as a pillar, in the configuration of FIG. 8B twice as many bit lines 820 can be accommodated, thereby doubling the data rate by increased parallel operation. The bit line pitch has been reduced to p=d/2, where d is the distance between the pillars in the dimension perpendicular to the bit lines. But since alternating pillars now underlie alternating bit lines 820, they can be decoded together by merging the SSLs 812-1 and 812-2 into a single SSL 813 as shown in FIG. 8C. This reduces the number of SSLs by half, thereby reducing disturbance, reducing power consumption and further improving data rate by reducing unit cell capacitance. Finally, with the merged SSL, the design rule limitations on spacing in the bit line dimension can be relaxed. Since there is no longer a requirement for spacing S or for the more severe edge gate rule G, the spacing between the two rows of pillars can be reduced in the bit line dimension as shown in FIG. 8D. The width of SSL 813 in the bit line dimension has been reduced accordingly as well.

The reduced height of the unit cell in the bit line dimension also results in a smaller unit cell area. In FIG. 8A, the area of the cell of FIG. 8A is AB· BD, which is larger than d² because the length of BD is larger than d. The cell area remains the same in FIGS. 8B and 8C. But because the height of the unit cell in the bit line dimension is shorter in FIG. 8D than in FIGS. 8A-8C, the area of the unit cell in FIG. 8D is smaller as well.

In a preferred embodiment, height of the unit cell in the bit line dimension is reduced until the spacing between pillars is constant on all sides of the parallelogram. That is, the lengths of line segments AB, BD, AC and CD are all equal to d, and the parallelogram is a rhombus. Still more preferably, the narrow angle of the rhombus is made equal to 60°, so that the length of BC is also equal to d. In this case the area of the unit cell is only (√ 3/2)*d², which is a minimum.

The technique of shifting rows of pillars in the direction perpendicular to the bit lines can be extended to shifting a larger number of rows by different amounts. For example, FIG. 9 illustrates three adjacent rows of pillars, each shifted by a distance d/3 relative to the immediately adjacent row of pillars. As a result, three times as many bit lines 820 can be accommodated, thereby greatly increasing parallel operation. The bit line pitch has been reduced to p=d/3, and a single merged SSL reduces the number of SSLs in the grid by ⅔, thereby further reducing disturbance, further reducing power consumption and further improving data rate by reducing unit cell capacitance. Finally, with the merged SSL, the design rule requirements for inter-SSL spacings S and dual gate thicknesses G have been reduced as well across the grid of pillars.

Similarly, FIG. 10 illustrates five adjacent rows of pillars, each shifted by a distance d/5 relative to the immediately adjacent row of pillars. As a result, five times as many bit lines 820 can be accommodated, thereby increasing parallel operation even further. The bit line pitch has been reduced to p=d/5, and a single merged SSL reduces the number of SSLs in the grid by ⅘, thereby still further reducing disturbance, still further reducing power consumption and still further improving data rate by reducing unit cell capacitance. Finally, with the merged SSL, the design rule requirements for inter-SSL spacings S and severity of the gate thicknesses design rule G have been reduced even further across the grid of pillars.

In general, the technique of shifting rows of pillars in the direction perpendicular to the bit lines can be extended to shifting rows by a distance d/n relative to the immediately adjacent row of pillars. As a result, n times as many bit lines can be accommodated, thereby greatly increasing parallel operation. The bit line pitch has been reduced to p=d/n, and a single merged SSL reduces the number of SSLs in the grid by (n−1)/n, thereby further reducing disturbance, further reducing power consumption and further improving data rate by reducing unit cell capacitance. Finally, with the merged SSL, the design rule requirements for spacings S and G are reduced as well across the grid of pillars.

Referring again to FIG. 9, it can be seen that the pillars in the grid are disposed laterally in such a way that in plan view they form a number of lines of pillars, indicated in the drawing as broken “pillar lines” 930-1, 930-2, 930-3 and 930-4 (collectively 930). (The pillar lines themselves are provided for visualization purposes, and do not represent any physical features on the device.) These lines are all parallel to each other and bear an acute angle θ>0° of intersection with the bit line conductors 920. In addition, it can be seen that each of the pillar lines has exactly 3 pillars within the region of one of the string select lines 912. All three pillars in each pillar line intersect a single common one of the string select lines 912.

Similarly, referring to FIG. 10, it can be seen that the pillars in the grid are disposed laterally in such a way that in plan view they form a number of lines of pillars, indicated in the drawing as pillar lines 1030-1, 1030-2, 1030-3 and 1030-4 (collectively 1030). These lines are all parallel to each other and bear a different acute angle θ>0° of intersection with the bit line conductors 1020. In addition, it can be seen that each of the pillar lines has exactly 5 pillars within the region of one of the string select lines 1012. All five pillars in each pillar line intersect a single common one of the string select lines 1012.

Again in the general case, for each value of n, the pillars in the grid are disposed laterally in such a way that in plan view they form a number of lines of pillars. These pillar lines are all parallel to each other and bear an acute angle θ>0° of intersection with the bit line conductors. In addition, each of the pillar lines has exactly n pillars within the region of one of the string select lines. All n pillars in each pillar line intersect a single common one of the string select lines.

Accordingly, shifting of rows of pillars in a dimension perpendicular to the bit lines permits both narrowing of the pitch of the bit line conductors, as well as widening of the SSLs. However, it is desirable that shifting distances be avoided which result in a narrowing of the pitch by greater than a factor of 10. This is because while the minimum pillar-to-pillar spacing design rules may remain satisfied, the design rules specifying a minimum spacing between bit line conductors may not. In addition, a bit line pitch narrowed by more than a factor of 10 might miss the process window as necessary to align with the pillars that the bit line is intended to superpose, or as necessary to miss the pillars that the bit line is intended to miss. These become significant risks for values of n>10. This limit on n is substantially independent of process shrinking, since the bit line pitch d/n shrinks by the same factor that the spacing d between pillars shrinks. Accordingly, it is preferable to limit n≦10. Note that n should be an integer.

As used herein, a given value is “responsive” to a predecessor value if the predecessor value influenced the given value. If there is an intervening processing element, step or time period, the given value can still be “responsive” to the predecessor value. If the intervening processing element or step combines more than one value, the signal output of the processing element or step is considered “responsive” to each of the value inputs. If the given value is the same as the predecessor value, this is merely a degenerate case in which the given value is still considered to be “responsive” to the predecessor value. “Dependency” of a given value upon another value is defined similarly.

As used herein, the “identification” of an item of information does not necessarily require the direct specification of that item of information. Information can be “identified” in a field by simply referring to the actual information through one or more layers of indirection, or by identifying one or more items of different information which are together sufficient to determine the actual item of information. In addition, the term “indicate” is used herein to mean the same as “identify”.

The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present invention may consist of any such feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

The foregoing description of preferred embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. For example, whereas embodiments herein are described using vertical channel charge storage memory cells, pillars with other types of memory cells can make use of aspects of the invention as well, albeit without necessarily achieving all of the benefits described above. In particular, and without limitation, any and all variations described, suggested or incorporated by reference in the Background section of this patent application are specifically incorporated by reference into the description herein of embodiments of the invention. In addition, any and all variations described, suggested or incorporated by reference herein with respect to any one embodiment are also to be considered taught with respect to all other embodiments. The embodiments described herein were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents. 

The invention claimed is:
 1. A memory device on a substrate, comprising: a multilevel stack of conductive layers, each of the layers being oriented parallel to the substrate; a plurality of pillars oriented orthogonally to the substrate, each of the pillars comprising a plurality of series-connected memory cells located at cross-points between the pillars and the conductive layers; a plurality of string select lines oriented parallel to the substrate and above the conductive layers, each of the string select lines intersecting a respective distinct subset of the pillars, each of the intersections of a pillar and a string select line defining a respective select gate of the pillar; and a plurality of parallel bit line conductors in a layer parallel to the substrate and above the string select lines, each of the bit line conductors superposing a respective distinct subset of the pillars, each of the pillars underlying one of the bit line conductors, wherein the pillars in the plurality of pillars are arranged on a regular grid having two lateral dimensions, the regular grid having a unit cell of four of the pillars A, B, C and D located at vertices of a parallelogram, pillar B being a pillar which is nearest to pillar A in the grid, and pillar C being a pillar which is non-collinear with pillars A and B but which is otherwise nearest pillar A in the grid, and wherein the parallelogram is non-rectangular and is oriented such that AB is perpendicular to the bit line conductors.
 2. The device of claim 1, wherein each of the memory cells comprises a vertical channel structure, a charge storage layer and an insulating layer.
 3. The device of claim 1, wherein the regular grid is such that every pair of pillars in the plurality of pillars, which pair are not aligned with each other in a dimension parallel to the bit line conductors, are separated in a lateral dimension orthogonal to the bit line conductors by at least d/10, where d is the length of AB.
 4. The device of claim 1, wherein all sides of the parallelogram have equal length.
 5. The device of claim 4, wherein the length of BC is the same as the length of AB.
 6. The device of claim 1, wherein the regular grid is such that every pair of pillars in the plurality of pillars, which pair are not aligned with each other in a dimension parallel to the bit line conductors, are separated in a lateral dimension orthogonal to the bit line conductors by a distance d/n, where d is the length of AB, and where n is an integer between 2 and 10 inclusive.
 7. The device of claim 1, wherein the string select lines comprise rectangles having a side oriented orthogonally to the bit line conductors, wherein each intersection of one of the string select lines and one of the bit line conductors uniquely identifies a single one of the pillars in the plurality, and wherein a particular one of the string select lines is sufficiently large in a dimension parallel to the bit line conductors that the particular string select line intersects at least pillars A and C of a particular one of the unit cells.
 8. The device of claim 7, wherein the particular string select line is sufficiently large in a dimension parallel to the bit line conductors so as to intersect all four of the pillars in a particular one of the unit cells.
 9. The device of claim 8, wherein all sides of the parallelogram have equal length.
 10. The device of claim 7, wherein all sides of the parallelogram have equal length.
 11. The device of claim 7, wherein the particular string select line is sufficiently large in a dimension parallel to the bit line conductors so as to intersect at least two pillars in different non-adjacent unit cells which are spaced from each other in a dimension not orthogonal to the bit line conductors.
 12. A memory device on a substrate, comprising: a multilevel stack of conductive layers, each of the layers being oriented parallel to the substrate; a plurality of pillars oriented orthogonally to the substrate, each of the pillars comprising a plurality of series-connected memory cells located at cross-points between the pillars and the conductive layers; a plurality of string select lines oriented parallel to the substrate and above the conductive layers and having a rectangular shape with a side oriented orthogonally to the bit line conductors, each of the string select lines intersecting a respective subset of the pillars, each of the intersections of a pillar and a string select line defining a respective select gate of the pillar; and a plurality of parallel bit line conductors in a layer parallel to the substrate and above the string select lines, each of the bit line conductors superposing a respective distinct subset of the pillars, each of the pillars underlying one of the bit line conductors, wherein the pillars in the plurality of pillars are disposed laterally such that in plan view they form a plurality of parallel lines of pillars, the lines of pillars having an acute angle θ>0° of intersection with the bit line conductors, each line of pillars having exactly n>1 pillars all intersecting a particular common one of the string select lines, and wherein in a direction perpendicular to the bit lines, for a length in the direction perpendicular to the bit lines which exceeds 2n of the bit lines, each n'th one of the bit lines intersects a pillar of a different one of the lines of pillars.
 13. The device of claim 12, wherein n≦10.
 14. The device of claim 12, wherein n is an integer between 2 and 10 inclusive.
 15. The device of claim 12, wherein each of the bit lines intersected by the parallel lines of pillars intersects exactly one of the pillars in each of a plurality of the lines of pillars.
 16. The device of claim 15, wherein all of the bit lines that intersect the particular string select line also intersect one of the pillars in one of the lines of pillars.
 17. The device of claim 12, wherein θ≠45°. 